Added contributing guidelines to CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Free-Certifications
+
+Thanks for wanting to improve this curated list — we appreciate contributions!
+
+How to contribute
+1. Search the repository to ensure a similar entry does not already exist.
+2. To add a new certification/course:
+   - Add a single row to the appropriate section in README.md following the existing table format:
+     | Technology | Provider | Description | Link | Expiration |
+   - Use full provider names (e.g., "Google Cloud Skills Boost", "AWS") and give a short one-line description.
+   - For Expiration, use ISO-ish date or "Unlimited" / "Unknown" / "Limited Time".
+3. If you're fixing links, please confirm the destination page returns 200 and contains course/exam info.
+
+PR checklist (please verify before submitting)
+- [ ] I added/updated a single entry in the README (or created a separate data file).
+- [ ] Link works and is not a redirect-to-paywall.
+- [ ] I removed placeholder text (no "[...]" in the submitted row).
+- [ ] I followed table formatting and added the provider consistently.
+- [ ] I signed the repository license or noted the content license if required.
+
+Style notes
+- Keep descriptions short (1–2 sentences).
+- If a provider has multiple entries, group logically and avoid duplicates.
+- Use "Unlimited" for courses that appear evergreen; use exact dates when given.
+
+Contact / Questions
+If in doubt, open an issue describing the change before making a large PR.